Routing refers to the problem of selecting a path through a computer network along which a message will travel from a source node to a destination node. Routing in multi-hop ad hoc networks presents particular challenges. Ad hoc networks are self-organizing networks in which nodes cooperatively maintain network connectivity. Nodes in an ad hoc wireless network are equipped with radio transceivers, such as network interface cards that conform to IEEE 802.11 standards. Such radios allow communication between nodes without requiring centralized network administration or fixed network infrastructure. Since each radio has a limited effective range, two distant nodes must communicate across multi-hop paths. Intermediary nodes in the path act as routers by forwarding packets.
Wireless community “mesh” networks are an increasingly important kind of multi-hop wireless network that is being deployed to provide broadband Internet access to communities. In these networks, nodes are either stationary or minimally mobile and typically do not rely on battery power. Rather than coping with mobility or minimizing power usage, routing techniques for wireless community networks focus on improving network capacity or the performance of individual transfers.
Suboptimal network capacity is a general problem associated with multi-hop wireless networks. In such networks, throughput degrades quickly as the number of hops increases. In 802.11 networks this is due in part to the inherent unfairness of the 802.11 MAC, which can stall the flow of packets over multiple hops. In addition, such networks typically use only a small portion of the available frequency spectrum and a single radio on each node for transmitting and receiving packets.
One approach to the improvement of network capacity for multi-hop wireless networks is to equip nodes with more than one radio. This solution has several advantages. A node with multiple radios can transmit and receive simultaneously. Moreover, a node having two radios can transmit on two channels simultaneously, enabling the network to make use of a greater portion of the frequency spectrum. In addition, the use of multiple heterogeneous radios that operate on different frequency bands (e.g., 802.11a at 5 Ghz and 802.11b/g at 2.4 GHz), with different bandwidth, range and fading characteristics, can improve robustness, connectivity and performance. Finally, 802.11 network interface cards are off-the-shelf commodity parts available at rapidly diminishing prices.
Most routing protocols for multi-hop wireless networks have focused on choosing paths having the least number of intermediate hops. The disadvantages of applying shortest-path routing in such networks are well-recognized. Choosing paths that minimize hop count can lead to poor performance, in part because such paths often include slow or lossy wireless links between distant nodes. Shortest-path routing techniques perform particularly poorly in networks having nodes with multiple radios. This is illustrated by the following two scenarios. First, consider a network in which each node has an 802.11a and an 802.11b radio. 802.11b radios generally have a longer range than 802.11a radios. Therefore, if shortest-path routing is used, most of the traffic in the network will be carried on the slower 802.11b links. Now suppose that each node in the network instead has two 802.11b radios, one tuned to channel 1 and the other tuned to channel 11. Consider a two-hop (three-node) path in this network. A path that is entirely over channel 1 or channel 11 will have significantly worse throughput than a path in which the two hops are on different channels. The use of a shortest-path algorithm that selects a path without ensuring that the hops are on different channels will therefore result in suboptimal performance.
Routing according to link quality is one well-known alternative to shortest-path routing. Several link quality metrics for multi-hop wireless networks have been proposed by researchers, but these schemes have focused on networks having homogeneous single-radio nodes. One example is the ETX metric, described in D. S. J. De Couto, D. Aguayo, J. Bicket, and R. Morris, “A High-Throughput Path Metric for Multi-Hop Wireless Routing,” ACM MOBICOM (September 2003), incorporated herein by reference. ETX measures the expected number of transmissions, including retransmissions, needed to send a unicast packet across a link. The path metric is the sum of the ETX values for each link in the path. The routing protocol selects the path with the minimum path metric.
A review of ETX is useful for understanding embodiments of the invention disclosed herein. The derivation of ETX begins with measurements of the underlying packet loss probability in both forward and reverse directions (denoted by pf and pr, respectively). First, the probability that a packet transmission is not successful is calculated. For a packet transmission to be successful, the 802.11 protocol requires successful acknowledgement of the packet. Let p denote the probability that the packet transmission from x to y is not successful. Then:p=1−(1−pf )*(1−pr)The 802.11 MAC will retransmit a packet for which a transmission was unsuccessful. Let s(k) denote the probability that the packet will be successfully delivered from x to y after k attempts. Then:s(k)=pk−1*(1−p)Finally, the expected number of transmissions required to successfully deliver a packet from x to y is denoted by ETX:
  ETX  =                    ∑                  k          =          1                ∞            ⁢              k        *                  s          ⁡                      (            k            )                                =          1              1        -        p            
While ETX performs well in homogeneous single-radio network environments, ETX will not necessarily select good routes in the multiple-radio scenarios discussed above. In the first scenario, in which each node has an 802.11a radio and an 802.11b radio, ETX will route most of the traffic on the 802.11b links, for two reasons. First, ETX only considers loss rates on the links, and not their bandwidths. Second, in an attempt to minimize global resource usage, ETX is designed to give preference to shorter paths over longer paths, as long as loss rates on the shorter paths are not significantly higher. In the second scenario, in which each node has two 802.11b radios, ETX is again likely to select suboptimal paths, because ETX does not give any preference to channel-diverse paths. Therefore, ETX, like other existing routing techniques and path metrics, fails to derive full benefit from the availability of multiple radios and the presence of link interference and varying bandwidths in multi-hop networks.